The present invention relates to a new and improved method of fabricating bevel and hypoid gear pairs by cutting out tooth spaces or gaps by means of an end or face-mill cutter head rotating about an axis of rotation while working in accordance with the imaginary mating gear method.
There are already known to the art methods of manufacturing gear pairs in accordance with the foregoing. With such heretofore known methods, for instance for fabricating one of the gears, the axis of the one gear is adjusted such that it meshes with an imaginary mating gear, also known as a generating gear or an imaginary generating gear, whose axis is concentric with a generating axis. The rotational axis of the end or face-mill cutter head is set perpendicular to a generatrix of a pitch surface of the gear, the generatrix confronting the end or face-mill cutter head. By rotating the end or face-mill cutter head about the axis of rotation there are thus cut-out the tooth spaces or gaps. In order to generate the tooth flanks, the axis of rotation of the face-mill cutter head performs an arcuate movement about the generating axis.
To produce the other gear or mating gear, the axis of the mating gear is positioned coaxially with respect to the generating axis. The axis of rotation of the face-mill cutter head is likewise adjusted perpendicular to a generatrix of the pitch surface of such mating gear. There is not required any subsequent generating of the tooth flanks.
In order to produce both gears, for instance a pair of bevel gears, the axis of rotation of the face-mill cutter head, in both instances, must be disposed perpendicular to a generatrix of the pitch cone of one gear. This means that the angle between the rotational axis and the generating axis is dependent upon the cone angle of the one gear. Since with state-of-the-art gear cutting machines for producing bevel gears the axis of rotation of the face-mill cutter head can only be tilted through a limited angle of inclination out of a parallel position towards the generating axis, the range of possible cone angles for the one gear of the bevel gear pair is limited, and specifically such that this limitation precludes small cone angles.
It is also known to fabricate only the one gear to be generated of a bevel or hypoid gear pair according to the imaginary mating gear method and to fabricate the mating gear of such gear pair by other, plunge-cut methods, e.g. by the imaginary crown gear method.
It is not possible or is only possible under extremely limited conditions to fabricate that gear of a gear pair which must be produced by generation in pairs of bevel gears with small shaft angles upon prior art machines according to the heretofore known imaginary mating gear method, because both of the gears of the bevel gear pair have small cone angles or angle of opening. The permissible operating range for known machines is well defined, e.g. it is defined for gear ratios as being not smaller than approximately 2.5:1 or it is defined for shaft angles as being about 65.degree. to 180.degree.. In other words, the cone angle or pitch angle of the imaginary mating gear may not be less than 90.degree. minus the maximum tilt angle of the cutter head spindle in the generating drum, which is about 30.degree. to 40.degree.. This means that gear transmissions or gear pairs having a cone angle below about 60.degree. cannot be fabricated in practice by the imaginary mating gear method.
Such gears which could not be produced according to the heretofore known imaginary mating gear methods, frequently could be fabricated according to the known imaginary crown gear method. Here, the axis either of the gear to be fabricated or of the mating gear is positioned such that in either case a generatrix of the pitch surface of the gear is disposed perpendicular to the generating axis and to the rotational axis which is parallel thereto, and thus, is located parallel to the end surface of the face-mill cutter head. Moreover, the generating axis intersects the center of an imaginary crown gear which extends parallel to the aforementioned generatrix. With this technique the teeth of the gear and the mating gear must be generated. This occurs in that the axis of rotation of the face-mill cutter head is rocked about the generating axis.
If in accordance with this method there can be manufactured gear pairs having random shaft angles or cone angles, then the diameter of the gears with small cone angles is extremely limited by the existing machines. This is so with regard to bevel gears because the center of the cone is located upon the generating axis and the height of the cone or the diameter of the generating crown gear depends upon the lateral spacing of the generating axis to the point of attack of the face-mill cutter head. With a given cone angle the diameter of the bevel gear is governed by this limited lateral spacing.
In particular, in Swiss Patent No. 594,468 there is taught an imaginary mating gear method for fabricating a pair of spur gears. With this method one of the elements, either the spur gear or the face-mill cutter head, performs an arc-shaped generating movement about a generating axis in order to fabricate the one spur gear, this generating movement coinciding with the gear axis of the mating gear. The mating gear is produced without any generating movement by a plunge cut method.
Hence, with this method there only can be fabricated spur gears, and specifically only spur gears whose axes or shafts extend parallel to one another. Significant in this regard are U.S. Pat. No. 4,183,703, granted Jan. 15, 1980 and U.S. Pat. No. 4,211,511, granted July 8, 1980.